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User’s Manual U12790EJ2V0UD
CHAPTER 21
STANDBY FUNCTION
21.1 Standby Function and Configuration
21.1.1 Standby function
The standby function is designed to decrease the power consumption of the system. The following two modes
are available.
(1) HALT mode
HALT instruction execution sets the HALT mode. The HALT mode stops the CPU operation clock, but the
system clock oscillator continues oscillating. In this mode, the current consumption cannot be decreased as
much as in the STOP mode. The HALT mode is effective for restarting immediately upon interrupt request
generation and to carry out intermittent operations such as in watch applications.
Although the CPU stops operating, the peripheral functions can operate. To lower the current consumption,
therefore, stop all unnecessary circuits before executing the HALT instruction.
(2) STOP mode
STOP instruction execution sets the STOP mode. In the STOP mode, the system clock oscillator stops and
the whole system stops. The CPU current consumption can be considerably decreased in this mode.
Data memory low-voltage hold (down to VDD = 2.3 V) is possible. Thus, the STOP mode is effective for holding
data memory contents with ultra-low current consumption.
If the supply voltage drops below 2.3 V, the system is reset by means of power-on clear reset. For reset, refer
to CHAPTER 22 RESET FUNCTION.
Because this mode can be released upon interrupt request generation, it enables intermittent operations to
be carried out.
However, because a wait time is necessary to secure the oscillation stabilization time after the STOP mode
is released, select the HALT mode if it is necessary to start processing immediately upon interrupt request
generation.
All the functions stop operating in this mode.
Some registers of the PLL frequency synthesizer and frequency counter are reset, but the other functions are
stopped with their current status held.
Cautions 1. When proceeding to the STOP mode, be sure to stop the peripheral hardware operations
before executing the STOP instruction.
2. The following sequence is recommended for power consumption reduction of the A/D
converter: first clear bit 7 (ADCS3) of ADM3 to 0 to stop the A/D conversion operation,
then execute the HALT or STOP instruction.